Many different orthopedic casting materials have been developed for use in the immobilization of broken or otherwise injured body limbs. Some of the first casting materials developed for this purpose involve the use of plaster of Paris bandages consisting of a mesh fabric (e.g., cotton gauze) with plaster incorporated into the openings and onto the surface of the mesh fabric.
Plaster of Paris casts, however, have a number of attendant disadvantages, including a low strength-to-weight ratio, resulting in a finished east which is very heavy and bulky. Furthermore, plaster of Paris casts typically disintegrate in water, thus making it necessary to avoid bathing, showering, or other activities involving contact with water. In addition, plaster of Paris casts are not air permeable, and thus do not allow for the circulation of air beneath the cast which greatly facilitates the evaporation and removal of moisture trapped between cast and skin. This often leads to skin maceration, irritation, or infection. Such disadvantages, as well as others, stimulated research in the orthopedic casting art for casting materials having improved properties over plaster of Paris.
A significant advancement in the an was achieved when polyisocyanate prepolymers were found to be useful in formulating a resin for orthopedic casting materials, as disclosed, for example, in U.S. Pat. No. 4,502,479 (Garwood et al.) and U.S. Pat. No. 4,411,262 (Von Bonin et al.). U.S. Pat. No. 4,502,479 sets forth an orthopedic casting material comprising a knit fabric which is made from a high modulus fiber (e.g., fiberglass) impregnated with a polyisocyanate prepolymer resin such as polyurethane. Orthopedic casting materials made in accordance with U.S. Pat. No. 4,502,479 provide significant advancement over the plaster of Paris orthopedic casts, including a higher strength-to-weight ratio and greater air permeability. However, such orthopedic casting materials tend not to permit tactile manipulation or palpation of the fine bone structure beneath the cast to the extent possible when applying a plaster of Paris cast. In this regard, knit fiberglass materials are not as compressible as plaster, and tend to mask the fine structure of the bone as the cast is applied, e.g., the cam provider may be limited in "feeling" the bone during reduction of the fracture. Although fiberglass fabrics are somewhat radiolucent they sometimes tend to mask the underlying bone structure to x-ray penetration. Oftentimes a fine mesh or a "shadow" can be seen on the x-ray image. This mesh, corresponding to the knitted fiberglass backing, obstructs the penetration of the x-rays and thereby obscures the free detail of the underlying bone on the x-ray image.
Fiberglass backings have further disadvantages. For example, fiberglass backings are comprised of fibers which have essentially no elongation. Because the fiber elongation is essentially nil, glass fabrics da not stretch unless they are constructed with very loose loops which can deform upon application of tension, thereby providing stretching of the fabric. Knitting with loosely formed chain stitches imparts extensibility by virtue of its system of interlocking knots and loose loops.
Like most knitted fabrics, fiberglass knits tend to curl or fray at a cut edge as the yarns are severed and adjacent loops unravel. Fraying and raveling produce unsightly ends and, in the case of an orthopedic cast, frayed ends may interfere with the formation of a smooth cast, and loose, frayed ends may be sharp and irritating after the resin thereon has cured. Accordingly, frayed edges are considered a distinct disadvantage in orthopedic casting tapes. Stretchy fiberglass fabrics which resist fraying are disclosed in U.S. Pat. No. 4,609,578 (Reed), the disclosure of which is incorporated by reference. Thus, it is well known that fraying of fiberglass knits at cut edges can be reduced by passing the fabric through a heat cycle which sets the yarns giving them new three-dimensional configurations based on their positions in the knit. When a fiberglass fabric which has been heat-set is cut, there is minimal fraying and when a segment of yarn is removed from the heat-set fabric and allowed to relax, it curls into the crimped shape in which it was held in the knit. Accordingly, at the site of a cut, the severed yarns have a tendency to remain in their looped or knotted configuration rather than to spring loose and cause fraying.
In processing extensible fiberglass fabrics according to U.S. Pat. No. 4,609,578 (Reed), a length of fabric is heat-set with essentially no tension. The fabric is often wound onto a cylindrical core so large batches can be processed at one time in a single oven. Care must be taken to avoid applying undue tension to the fabric during wind-up on the knitter which would distort the knots and loops. To prevent applying tension to the fabric during winding, the winding operation is preferably performed with a sag in the fabric as it is wound on the core.
Alternatively, U.S. Pat. No. 5,014,403 (Buese) describes a method of making a stretchable orthopedic fiberglass casting tape by knitting an elastic yarn under tension into the fiberglass fabric in the length direction, releasing the tension from the elastic yarn to compact the fabric and removing the elastic yarn from the fabric. The elastic yarn is removed by a combustion process which may cause localized areas of high temperature which may degrade the fiberglass yarns. The physical properties of glass fibers are adversely affected when subjected to temperatures in excess of about 540.degree. C. Heating fiberglass fabrics to temperatures above about 540.degree. C. should be avoided as subjecting the fiberglass to temperatures of greater than about 540.degree. C. can weaken the fiberglass yarns in the fabric which may result in reduced strength of casts made from such fabrics.
From the foregoing, it will be appreciated that what is needed in the art is an orthopedic casting material which has both the advantages of plaster of Paris, e.g., good moldability and palpability of the free bone structure, and the advantages of non-plaster of Paris materials, e.g., good strength-to-weight ratio and good air permeability. In this regard it would be a significant advancement in the art to provide such a combination of advantages without actually using plaster of Paris, thereby avoiding the inherent disadvantages of plaster of Paris outlined herein. It would be a further advancement in the art to provide such non-plaster of Paris orthopedic casting materials which have as good or better properties than the non-plaster of Paris orthopedic casting materials of the prior art. Such orthopedic casting materials and methods for preparing the same are disclosed and claimed herein.